webpack: replace DefinePlugin w/ EnvironmentPlugin

the new version of webpack has a new plugin (EnvironmentPlugin) that
makes the syntax for doing this same thing a little more concise

test plan:
* webpack should work the same as it used to

Change-Id: I7116cc26386283c84a3d82fbbbd06f5497829a1c
Reviewed-on: https://gerrit.instructure.com/133390
Tested-by: Jenkins
Reviewed-by: Brent Burgoyne <bburgoyne@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>

frontend_build/baseWebpackConfig.js

@@ -246,9 +246,7 @@ module.exports = {
     // sets these environment variables in compiled code.
     // process.env.NODE_ENV will make it so react and others are much smaller and don't run their
     // debug/propType checking in prod.
-    new webpack.DefinePlugin({
-      'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV),
-    }),
+    new webpack.EnvironmentPlugin(['NODE_ENV']),
 
     new WebpackCleanupPlugin({
       exclude: ['selinimum-manifest.json']